IUE Observations Summary: Update 21 July 1994.

European space scientists cointinue with their American colleagues in the extensive observing program with the International Ultraviolet Explorer Space Observatory (IUE). The unique nature of IUE, which is a truly international collaborative Project (ESA/NASA/UK), has made it possible to take the first spectra only 20 minutes after the first plumes of impact A had risen beyond the limb of Jupiter, and before they were dispersed by the strong stratospheric winds of Jupiter. These spectra are of particular interest, because they are taken at ultraviolet wavelengths, a range which allows to study the upper layers of the Jupiter atmosphere (hundreds of km above the visible clouds). Such results can only be obtained from space since the ultraviolet range is inaccessible to ground based observatories.

Observations of the Jupiter system have been performed with the IUE satellite on a 24 hour basis since the 15th of July, and intermittantly since early June. The IUE imaging spectrographs have a combined spectral coverage from 1150- 3300Å at resolutions from .14 to 6Å. We are monitoring several of the best studied upper atmospheric features of Jupiter and its magnetosphere including the Aurora, the Ly-alpha equatorial anomaly, and the Io torus. We have detected changes in some of these features (in particular, the aurora has been weak) that we attribute to (or at least find suspiciously coincidental with) the effects of the comet fragment impacts and/or to the passage of dust through the inner Jovian magnetosphere.

The IUE is also being used to monitor the development of spectral features in the impact areas, and in particular has been useful for showing the timescales that describe the development of the dark features seen in the WFPC-II images. The A, B, E, G, K, and Q impacts have been observed wiht great success.

Spectra of plumes associated with impacts

The spectra taken just after the impact of comet fragment A have shown a clear increase in the reflectivity of the Jupiter atmosphere by some 20% compared to normal. Only two hours after the first spectrum (and less than 2.5 hours after the impact) the UV brightness of the plume had already decreased considerably, and the ultraviolet reflectivity was very close to its normal value. Dr. Walt Harris (Un. of Michigan, USA), also a memeber of the ESA/NASA IUE Science Team reports that the analysis of spectra obtained on the approaching and receeding limb while we followed the G impact site with the IUE small aperture show that the region experienced a 50% drop in reflectivity as it rotated moved across the disk of the planet. Many possible absorption and emission features have also been observed in spectra obtained from the impact sites.

Comparison with spectra taken under similar conditions before the impact showed new absorption lines at wavelengths of 176 and 178 nm with a possible localized emission near 182 nm and 330 nm Both features were short-lived and had disappeared after a few hours. The identification of the absorption lines with specific atoms or molecules has not been possible yet because of the difficulty in the identification of molecular lines of carbohydrates, many of which show lines in this wavelength range. Dr. Claude Emerich (IAS, Orsay) who is currently making the IUE observations at the ESA IUE Observatory in Spain suggests that one possible identification of the 182 nm emission line could be the emission of atomic sulfur, suggesting that the plume consists of the exploded material of the cometary nucleus and the destruction of the Comet nuclei takes place well above the Jupiter cloud layer. This would also be suggested by the tentative identification of the 330 nm emission with the unexpected presence of a possibly Sodium (Na).

Absence of reflection pulses of the impact flash

Shortly before the spectra, observations are made with IUE to detect the predicted brightening of the Jovian satellites in response to the flash of the direct impacts, which are all taking place on the backside of Jupiter. Europa was used as a mirror to look behind Jupiter. No noticeable brightening at the 1% level was observed until now. This will considerably constrain the possible models for the phenomena associated with the direct impact.

Ly-alpha Imaging of Impact Plumes

Dr. Gilda Ballester (Univ. Of Michigan, USA) reports for the IUE ESA/NASA observing team the likely detection of Lyman alpha emission of atomic Hydrogen off the Jovian limb associated with the plume of the impacts of fragments K, and P2 using the modest imaging capabilities of the IUE Space Observatory satellite.

Although the Point-Spread-Function (i. e. spatial resolving power) of IUE at Lyman-alpha is 4-5 arcsec and IUE has no two dimensional direct imaging capabilities , the emission can be seen to peak near/at the limb. There is also quite extended,weaker emission within the large (9x21 arcsec) aperture, extending possibly out to 3-4 arcsec above the limb. Molecular hydrogen emission has also been detected (possibly less extended than the atomic H-Lyman alpha, although this needs to be confirmed by a more extensive analysis later on). Also impacts Q and R have also been observed but not imaged with the small (3 arcsec) aperture.

The ESA/NASA studies with IUE will continue to monitor the effects of the impacts on a 24 hour basis with the IUE until the end of the week, and then on a less regular schedule until it becomes unobservable with the IUE Space Observatory on Aug. 15.